1. Field of the Invention
The invention relates to a method and apparatus for performing color space conversion, more particularly to a method and apparatus requiring a fewer number of look-up tables for converting digitized luminance-chrominance color space signals to digitized RGB color space signals.
2. Description of the Related Art
It is desirable to merge a video signal with graphic signals in a multi-media computer system. The video signal may come from a television image processing system having a capture or frame grabbing capability, or from a compressed video playback of a CD-ROM or network transmission. Color space conversion is needed in image processing applications to convert luminance-chrominance color space signals, which offer the advantages of a lower transmission bandwidth and a lower data storage requirement, into RGB color space signals, which are used when displaying an image on a computer monitor.
CCIR 601, which was proposed by the Comite Consultatif International des Radiocommunications (CCIR), establishes the following formulas for converting from the YCbCr luminance-chrominance color space to the RGB color space: EQU R=Y+1.402(Cr-128) (a.1) EQU G=Y-0.714(Cr-128)-0.344(Cb-128) (a.2) EQU B=Y+1.772(Cb-128) (a.3)
If U and V are used to represent the shifted chrominance components (Cb-128) and (Cr-128), respectively, Equations (a.1) to (a.3) can be rewritten as follows: EQU R=Y+1.402V (b.1) EQU G=Y-0.714V-0.344U (b.2) EQU B=Y+1.772U (b.3)
where Y ranges between 0, 255!, and U and V range between -128, 127! in an 8-bit representation for each of the Y, Cb and Cr color space components.
Color space conversion is often implemented by employing multipliers or look-up tables to achieve the matrix multiplication operations. Look-up tables are preferred because of their less complicated constructions. It is noted that the matrix multiplication operations dominate the hardware complexity of a color space converting apparatus. As such, the number of look-up tables is critical in determining the cost of implementing the color space converting apparatus. To implement the YCbCr to RGB color space conversion of Equations (a.1) to (a.3), a conventional color space converter usually requires four look-up tables to perform the matrix multiplication of chrominance components. Although the use of four look-up tables is less expensive to implement as compared to another conventional color space converter which uses a 3-by-3 multiplication matrix, a further reduction in the number of look-up tables is desirable.